Formation of semi-coherent Zr-rich lamellar phase in 2:17-type Sm-Co-Fe-Cu-Zr magnets with high Fe content

Abstract

The 2:17-type Sm-Co-Fe-Cu-Zr high-temperature magnets exhibit unique cellular nanostructure, with Zr-rich lamellar phase (Z-phase) penetrating through the 2:17R nanocells and 1:5H cell boundaries. In this work, a face-centered cubic (FCC) Zr-rich lamellar phase was identified in an Fe-rich Sm25Co42.9Fe23.5Cu5.6Zr3.0 (wt.%) magnet. The orientation relationship between FCC and 2:17R phases is [100]-2:17 ∥ [11¯0]-FCC and (001)-2:17R ∥ (111)-FCC. Unlike the previously-reported hexagonal or rhombohedral Z-phases that are fully coherent with 2:17R or 1:5H phases, both the broad interphase (111)-FCC/(001)-2:17R and the (224¯)-FCC/(010)-2:17R interphase at the peripheries of the FCC Z-phase exhibit semi-coherent morphology, due to the large lattice mismatch. The semi-coherent FCC Z-phase can be attributed to the thickening of the coherent Z-phase that gradually loses coherency, as a result of a gradual reduction in elastic strain energy and interfacial energy. This finding may add new insights into the solid-state phase transitions of Sm-Co-based magnets.